Chapter 5 - MODBUS-RTU Communication
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Example) Command Message (Request) for reading one address from address ‘3000’
ENQ Drive No. CMD Address
The Number
of address to
read
SUM EOT
05h “01” “R” “3000” “1” “A7” 04h
1 byte 2 bytes 1 byte 4 bytes 1 byte 2 bytes 1 byte
SUM = ‘0’ + ‘1’ + ’R’ + ‘3’ + ‘0’ + ‘0’ + ‘0’ + ‘1’
= 30h + 31h + 52h + 33h + 30h + 30h + 30h + 31h
= 1A7
h
5.6.3 Detail Communication Protocol
Request for Read: Request for read ‘n’ numbers of WORD from address ‘XXXX’.
ENQ Drive No. CMD Address
The number
of address to
read
SUM EOT
05h
“01” ~
“1F”
“R” “XXXX”
“1” ~ “8” =
n
“XX” 04h
1 byte 2 bytes 1 byte 4 bytes 1 byte 2 bytes 1 byte
Total byte = 12 bytes
The quotation marks (“ ”) mean character.
Acknowledge Response
ACK Drive No. CMD Data SUM EOT
06h “01” ~ “1F” “R” “XXXX” “XX” 04h
1 byte 2 bytes 1byte N * 4 bytes 2 bytes 1 byte
Total byte = 7 + n * 4 = max. 39 bytes
Negative Acknowledge Response
NAK Drive No. CMD Error Code SUM EOT
15h “01” ~ “1F” “R” “**” “XX” 04h
1 byte 2 bytes 1 byte 2 bytes 2 bytes 1 byte
Total byte = 9 bytes
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